The present invention relates to a driving circuit apparatus, and more particularly to applying to drive the cathode ray tube (CRT), which is especially required high resolution.
In conventional cathode ray tube (CRT) display device to be used for computer aided design (CAD), it is necessary to display high resolution image on the tube compared with an ordinary CRT display device.
To display high resolution image in the CRT display device for the computer aided design, a high frequency video signal, 150 [MHz] as against an ordinary video signal of 10 [MHz], is supplied to the display device via the voltage output type image output circuit 1A (FIG. 1) and the current output type image output circuit 1B (FIG. 2).
More specifically, the image output circuit 1A amplifies an input video signal V.sub.in quintuple by the first step pre-drive amplifier 2 and further amplifies fifteen-fold at the outer attached cathode drive amplifier 3, and supplies to the CRT 4 as an output image signal V.sub.out.
In a case when an input video signal V.sub.in with a peak value of 0.6 [V.sub.pp ], showing maximum amplitude from positive side wave crest value to negative side wave crest value, is inputted, it is necessary that the input video signal V.sub.in is amplified to 3.0 [V.sub.pp ] when outputted from the pre-drive amplifier 2, and to be outputted to the CRT 4 as approximately 45 [V.sub.pp ] output image signal V.sub.out via the cathode drive amplifier 3.
In case of amplifying this input video signal V.sub.in at the voltage output type pre-drive amplifier 2 (FIG. 1), the cathode drive amplifier 3 supplies a video signal V1 to be outputted from the pre-drive amplifier 2, to the base of transistor Q1 via the buffer circuit 5.
Here, said transistor Q1 is a NPN type transistor and is constituted by an emitter follower amplifying stage as well as resistance R1.
A collector of said transistor Q1 is cascade connected to a transistor Q2 which constitutes constant current source to compensate merits in high frequency band of the transistor Q1 and for having higher pressure proof for high voltage power source.
A collector of the transistor Q2 is connected to load resistance R2 connected to high voltage power source of 60 [V], and is arranged to output an output image signal V.sub.out from the connecting node PO of the collector and load resistance R2 via the buffer circuit 6.
In case of amplifying the input video signal V.sub.in at the current output type pre-drive amplifier 2 (FIG. 2), the cathode drive amplifier 3 supplies electric current outputted from the pre-drive amplifier 2 to an emitter of transistor Q3, and outputs the output image video signal v.sub.out from the connecting node P1 of collector connected load resistance R3 and the transistor Q3 via the buffer circuit 7.
At this time, the base of driving transistor Q3 is supplied with reference power source V.sub.ref and large electric current of 150 [mA.sub.pp ] flows to load resistance R3.
In thus constructed image output circuit 1A (FIG. 1), if the CRT 4 is driven by high frequency video signal V.sub.in, such as 150 [MHz], a signal amplitude of 45 [V.sub.pp ] is necessary at an output terminal of the cathode drive amplifier 3. Thus, a signal amplitude of 3.5 [V.sub.pp ] is needed at an output terminal of the pre-drive amplifier 2, and at this time, a slewing rate SR of the video signal V.sub.in to be outputted from the pre-drive amplifier 2 becomes 2 [V/ns], 1,000 times more than an ordinary slewing rate SR, is necessary.
However, since it is necessary to run much electric current and increase driving voltage to satisfy such a high slewing rate SR, it was difficult to have the voltage output type pre-drive amplifier 2 built in the IC, which is unable to run much electric current, when inputting a video signal of over 100 [MHz] frequency for high resolution.
In case of driving the CRT 4 with 150 [MHz] signal V.sub.in via the image output circuit 1B as shown in FIG. 2, provided that a resistance value of the load resistance R3 connected to the output terminal is 300 [.OMEGA.], it is necessary to drive control the driving transistor Q3 running 150 [mA] (=45 [V]/300[.OMEGA.]) electric current at the output stage of pre-drive amplifier 2. However, if the pre-drive amplifier 2 is built in the IC, it is difficult to control such big electric current as 150 [mA] because an electric current capacity in the IC is as small as about 1 [mA]. Therefore, in case of driving the CRT 4 which requires high resolution, it was difficult to have the pre-drive amplifier 2 built in the IC.